1. Field of the Invention
The present invention relates to an ultra-light sound insulator that prevents propagation of noise and other undesired sound from an engine room or any other vehicle exterior into a vehicle interior. More specifically the invention pertains to an ultra-light sound insulator that is extremely light in weight and effectively absorbs noise and other undesired sound to prevent their propagation into the vehicle interior.
2. Description of the Related Art
Patent Document 1 discloses a multifunctional kit (41), which is used in vehicles to attain noise reduction and heat insulation and more specifically to have sound-absorbing, sound-insulating, oscillation-damping, and heat-insulating effects on floor insulation, end wall insulation, door covering, and roof inner covering. The multifunctional kit (41) includes at least one areal vehicle part (11) and a multi-layer noise-reducing assembly package (42). The assembly package (42) has at least one porous spring layer (13), which is preferably formed from an open-pored foam layer. An air gap (25) is interposed between the assembly package (42) and the areal vehicle part (11). The multi-layer assembly package (42) does not have a heavy-weight layer to give the ultra-light kit (41) suitable for the optimum combination of sound-insulating, sound-absorbing, and oscillation-damping properties. The assembly package (42) also has a micro-porous stiffening layer (14), which preferably consists of an open-pored fiber layer or fiber/foam composite layer. The micro-porous stiffening layer (14) has a total airflow resistance of Rt=500 Nsm−3 to Rt=2500 Nsm−3, in particular of Rt=900 Nsm−3 to Rt=2000 Nsm−3, and an area-weight (weight per unit area) of mF=0.3 kg/m2 to mF=2.0 kg/m2, in particular of mF=0.5 kg/m2 to mF=1.6 kg/m2. The advantages of this kit are particularly evident with the application of thin steel sheeting, light aluminum sheeting, or organo-sheeting, as is favorably used today in the automobile industry. A further advantage of this kit lies in the extremely low heat conductivity of the applied porous spring layer, which leads to the fact that this kit apart from its good acoustic effectiveness (sound insulation effects) also has good heat insulation.
Patent Document 2 discloses a sound insulator 10 for vehicles. In this prior art sound insulator 10, a first air-permeable sound absorption layer 20, an air-impermeable sound insulation layer 30, and a second air-permeable sound absorption layer 40 are arranged in this order from a vehicle interior 100. The first air-permeable sound absorption layer 20 does not have an air-impermeable layer on the side of the vehicle interior, while the second air-permeable sound absorption layer 40 does not have an air-impermeable layer on the opposite side of the vehicle interior. The sound insulator is light in weight and is designed to effectively reabsorb noise, which has passed through the sound insulator and leaked into the vehicle interior, and to effectively absorb noise incoming from a site other than an engine room into the vehicle interior.
Patent Document 3 discloses an automobile insulator (20) attached to the vehicle interior side of a vehicle body panel (10). The insulator (20) has a mono-layer sound absorption layer (21), the base of which is a fiber molded object. The insulator (20) is constructed as an air-permeable insulator to absorb the noise, which is propagated through the vehicle body panel (10) and enters the sound absorption layer (21), while absorbing reflected noise, which is transmitted through the sound absorption layer (21), is reflected from the inner face of a vehicle interior panel (40), and enters again the sound absorption layer (21) from the surface side. A surface layer (22) of high-density fibers set to have a higher density than the area density of the sound absorption layer (21) is formed on at least one of the surface and the rear face of the sound absorption layer (21). A surface layer (27) of a foamed resin sheet material is also formed to wholly or partly cover at least one of the surface and the rear face of the sound absorption layer (21). This structure excludes the conventional sound insulation layer to reduce the weight, while preventing an increase in sound pressure in the instrument panel (40) to enhance the stillness in the vehicle interior.
Patent Document 4 discloses a laminated object obtained by integrally forming a polyolefin resin foam having a skin peel strength of not greater than 20 N/cm and an L value of not higher than 60 and a bulky non-woven fabric having a thickness of not less than 5 mm and a density of not higher than 50 kg/cm3. The laminated object has an area-weight of not greater than 3 kg/m2. The laminated object is light in weight and easily shaped and has high recyclability and good appearance.
A dash silencer including a surface layer and a sound absorption layer (Patent Document 1+Patent Document 3) has been proposed by taking advantage of the air-flow resistance.
The transmission loss and the sound absorption power of the conventional sound insulation structure are compared with those of the structure disclosed in Patent Document 1. In this discussion, a low frequency domain includes 315 Hz and lower as the ⅓ octave band center frequency. A medium frequency domain includes 400 to 1600 Hz. A high frequency domain includes 2000 Hz and higher.
The transmission loss and the sound absorption power of the conventional sound insulation type structure (see FIG. 27, hereafter referred to as the ‘structure of FIG. 27’) are compared with those of the structure disclosed in Patent Document 1 (see FIG. 28, hereafter referred to as the ‘structure of FIG. 28’).
The dash silencer having the structure of FIG. 27 has an area-weight of 6.0 kg/m2, whereas the structure of FIG. 28 has a currently available effective area-weight of 2.0 kg/m2. These products are attached to a vehicle body panel, which has an area-weight of 6.2 kg/m2.
According to the transmission loss curve of FIG. 29(a), the structure of FIG. 27 has the greater transmission loss than the weight law. This is ascribed to the double-wall structure of the air-impermeable surface layer and the panel and the presence of the intermediate sound absorption material having the air-flow resistance. The high area-weight of rubber sheet, however, causes a significant transmission resonance in a low frequency domain to drastically lower the transmission loss.
According to the transmission loss curve of FIG. 29(a), the structure of FIG. 28 has the smaller transmission loss than the weight law. The structure of FIG. 28 has also a double-wall structure of the air-permeable surface layer and the panel, however, the surface layer lets air through, and this causes sound leakage in a high frequency domain. The structure of FIG. 28 accordingly does not give sufficient transmission loss for sound insulation.
According to the sound absorption rate curve of FIG. 29(b), the structure of FIG. 27 has a peak of sound absorption rate, due to strong surface resonance, in a low frequency domain, while only little or substantially no sound absorption rate in the medium to high frequency domain.
According to the sound absorption rate curve of FIG. 29(b), the structure of FIG. 28 takes advantage of the surface resonance of the surface layer having the high air-flow resistance and the sound absorbing power of the rear sound absorption layer to attain sound absorption power in the medium to high frequency domain.
The indirect noise, which incomes from everywhere of the automobile and is reflected, rather than the direct noise, which directly incomes from the dash panel to the dash silencer, more significantly affects the actual stillness in the vehicle interior. The structure of Patent Document 1 has significantly lowered transmission loss but relatively higher sound absorbing power in a medium to high frequency domain, thus attaining somewhat equivalent stillness in the vehicle interior to the effects of the conventional structure. The structure of Patent Document 1, however, has an advantage of significant weight reduction of a product and has favorably been applied to the recent dash panel structure.    [Patent Document 1] Patent Publication Gazette No. 2000-516175    [Patent Document 2] Patent Laid-Open Gazette No. 2001-347899    [Patent Document 3] Patent Laid-Open Gazette No. 2002-220009    [Patent Document 4] Patent Laid-Open Gazette No. 2002-347194
The automobile of some vehicle structure has large effects of direct noise. The structure of FIG. 28 may give an insufficient transmission loss (see FIG. 29(a)) and thus fail to attain the required stillness in the vehicle interior. Additionally, actual products have designed patterns and varying thickness of the sound absorption layer in a range of 1 to 30 mm. The dash silencer having the structure of FIG. 28 disclosed in Patent Document 1 takes advantage of the sound absorbing power of the sound absorption layer in the high frequency domain. Reduction in thickness of the sound absorption layer thus results in the lowered sound absorbing power. Additionally, the sound absorption layer is made of felt having a thickness in a range of 30 to 50 mm. The thin wall portion has the lowered air-flow resistance than the other wall portion and does not give the sufficient sound absorbing power. The dash silencer having the structure disclosed in Patent Document 1 that assures the stillness in the vehicle interior due to sound absorbing power may thus not exert sufficient performances.
The prior art sound insulator is designed to reduce the noise directly incoming from the vehicle exterior and have good sound absorbing power in a wide frequency domain, while not having the sufficient countermeasure to absorb reflected noise in the vehicle interior. As shown in FIG. 30, a ⅓ octave band center frequency domain of 800 to 1600 Hz is essential for the clearness of conversion. The prior art structure has insufficient sound absorption effects at the frequency of about 1000 Hz, which is important for cleanness of conversation.
In the sound insulator of Patent Document 2, the sound absorbing power of the sound absorption material is used to absorb sound in a frequency domain of not lower than 1000 Hz, as shown in FIG. 31. The reduced thickness of the sound absorption layer thus tends to lower the sound absorption rate.
The sound insulator having the structure of FIG. 28 functions to absorb reflected sound in the vehicle interior, but there is no clear method of regulating the sound absorption frequency.
The prior art sound insulators disclosed in Patent Documents 3 and 4 have not given any consideration to the effects of the restricting state at the interface between the sound absorption layer and the surface layer and the air permeation of the surface layer on the sound absorption characteristics and the sound insulation characteristics. Actual products have complicated shapes and require the interfacial adhesion strength. The prior art sound insulators may thus have different sound absorption and sound insulation characteristics from designed conditions and may not be usable in limited spaces.